While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 μm or less.
The chemically amplified resist materials for use in photolithography using light of an excimer laser, especially ArF excimer laser having a wavelength of 193 nm, are, of course, required to have a high transparency to light of that wavelength. In addition, they are required to have an etching resistance sufficient to allow for film thickness reduction, a high sensitivity sufficient to eliminate any extra burden on the expensive optical material, and especially, a high resolution sufficient to form a precise micropattern. To meet these requirements, it is crucial to develop a base resin having a high transparency, rigidity and reactivity. Active efforts have been made to develop such base resins.
Known high transparency resins include copolymers of acrylic or methacrylic acid derivatives (see JP-A 4-039665). Since these copolymers allow the amount of acid labile units to be increased as desired, it is relatively easy to enhance the acid reactivity of high molecular weight copolymers.
Introduction of more acid labile units is advantageous in achieving a higher sensitivity and contrast, but gives rise to the problem of exaggerated pattern density-dependent size difference. Namely, even when an identical quantity of exposure is made through a mask with an identical feature size, the feature size of the resulting pattern becomes significantly smaller in densely packed line areas than in sparsely packed line areas. This suggests a contradiction that a larger amount of acid labile units sufficient to provide a higher sensitivity and resolution is unacceptable for practical use because of exaggerated pattern density-dependent size difference whereas a smaller amount of acid labile units provides an acceptable level of pattern density-dependent size difference, but impractical levels of sensitivity and resolution.